The quickest way to get this is to fire up a siege mode match on the Catacombs with two players. Whoever is the agent first runs up to the NPC agent and gets him to follow him with the "come here" command (hold B and press down on the D-Pad). Then the agent leads the NPC over to the spike trap in the hallway near the back of the level. The creature then operates the lever when the NPC is over the spike trap, killing the NPC instantly. When the round switches, just reverse the roles. Overall it takes about 20 seconds or so per kill.
Some notes, make sure that whoever goes first kills the agent when there is at least 2:30:00 left on the time, if you kill him too quickly, there won't be enough time left for the next person to do it. Also, make sure the NPC is in the middle of the spikes when you activate them, if he is near the edge, he will fly out of them instead of dying instantly.